There has been a rapid growth in the market for linear low density polyethylene (LLDPE) particularly resin made under mild operating conditions, typically at pressure of 100 to 300 psi and reaction temperatures of less than 100.degree. C. This low pressure process provides a broad range of LLDPE products for film, injection molding, extrusion coating, rotational molding, blow molding, pipe, tubing, and wire and cable applications. LLDPE has essentially a linear backbone with only short chain branches, about 2 to 6 carbon atoms in length. In LLDPE, the length and frequency of branching, and, consequently, the density, is controlled by the type and amount of comonomer used in the polymerization. Although the majority of the LLDPE resins on the market today have a narrow molecular weight distribution, LLDPE resins with a broad molecular weight distribution are available for a number of applications.
LLDPE resins designated for commodity type applications typically incorporate 1-butene as the comonomer. The use of a higher molecular weight alpha-olefin comonomer produces resins with significant strength advantages relative to 1-butene copolymers. The predominant higher alpha-olefins in commercial use are 1-hexene, 1-octene, and 4-methyl-1-pentene. The bulk of the LLDPE is used in film products where the excellent physical properties and drawdown characteristics of LLDPE film makes this film well suited for a broad spectrum of applications. Fabrication of LLDPE film is generally effected by the blown film and slot casting processes. The resulting film is characterized by excellent tensile strength, high ultimate elongation, good impact strength, and excellent puncture resistance.
These properties together with toughness are enhanced when the polyethylene is of high molecular weight. However, as the molecular weight of the polymer increases, the processability of the resin usually decreases. By providing a blend of polymers, the properties characteristic of high molecular weight resins can be retained and processability, particularly extrudability, can be improved.
Three major strategies have been proposed for the production of resins of this nature. One is post reactor or melt blending, which suffers from the disadvantages brought on by the requirement of complete homogenization and attendant high cost. A second is the direct production of resins having these characteristics via a single catalyst or catalyst mixture in a single reactor. Such a process would provide the component resin portions simultaneously in situ, the resin particles being ultimately mixed on the subparticle level. In theory, this process should be the most rewarding, but, in practice, it is difficult to achieve the correct combination of catalyst and process parameters necessary to obtain the wide diversity of molecular weights required. The third strategy makes use of multistage reactors, the advantage being that a quite diverse average molecular weight can be produced in each stage, and yet the homogeneity of the single reactor process can be preserved. Furthermore, two or more reactors running under their own set of reaction conditions permit the flexibility of staging different variables. To this end, many versions of multistage reactor processes have been offered, but optimization has been elusive.